1. Technical Field
The present invention pertains to the field of wireless communication. More particularly, the present invention pertains to criteria used in inter-frequency or inter-system handover in cellular communication.
2. Discussion of Related Art
TD-SCDMA (Time Division-Synchronized Code Division Multiple Access), also known as UTRA (Universal mobile telecommunications system (UMTS) Terrestrial Radio Access) TDD (time division duplex) 1.28 Mcps option, is a so-called 3G (third generation) wireless communication system that is supposed to be used for example in China, possibly along with two other 3G technologies—UTRA FDD version of WCDMA (Wideband CDMA) and CDMA2000. A TD-SCDMA system is therefore sometimes indicated as a TDD SCDMA system, and is also known as a Low Chip Rate (LCR) TDD system.
TD-SCDMA uses both TDMA (Time Division Multiple Access) and CDMA as multiple access methods, which means there are several possible slots for each transmitting direction on a single carrier, and in each slot one or more users, separated by orthogonal codes, can transmit or receive data simultaneously, one user can even occupy more than one timeslot during multi-slot mode.
Since TD-SCDMA uses a 1.6 MHz bandwidth, it has a low single-carrier capacity. Thus, a TD-SCDMA is usually implemented (by a network operator) using more than one carrier. Even so, to prevent a communication link from deteriorating because of over-capacity, inter-frequency handover (IFHO) must be performed relatively frequently. IFHO is also performed because of coverage, i.e. because of a user equipment (UE) (a cellular communication device, e.g. a cellular telephone) moving beyond the coverage of a network service access point (SAP), e.g. a so-called Node B.
TD-SCDMA uses hard handover (from one SAP to another, without a time interval during which the UE is in communication with both) instead of soft handover (includes a time interval during which the UE is in communication with two or more SAPs) to make the UE always connect to a single best cell. The quantity used to define how good a cell is, per the prior art, P-CCPCH RSCP (Primary-Common Control Physical CHannel Received Signal Code Power). After passing the cell-planning phase, the coverage area is decided by the antenna down tilt, gain map and Tx (transmit) power of P-CCPCH. The Tx power of P-CCPCH changes only rarely. Hence the coverage area of a cell also rarely changes. The cell-planning phase ensures that the best cell's P-CCPCH RSCP in the whole service area is above an acceptable threshold.
Based on where the UE is located in a service area, the P-CCPCH RSCP of all detectable cells would reflect the admission cost (power increase of the whole network for both directions) of the UE in that location. Estimation cost in slot 1 to slot 6 from measurement of slot 0, which is the slot used by P-CCPCH, would not be very accurate, but it would be sufficient on an average basis without information about other slots in the location. The potential downlink power increase after admitting the UE in the specific location is decided by both the path loss from the serving cell and the path loss from any interfering cells. The serving cell's P-CCPCH RSCP would only directly reflect the path loss from the serving cell, but it would also indirectly reflect the path loss from interfering cells. In other words, the serving cell's P-CCPCH RSCP is affected by and so indicates interference from interfering cells. So selecting a cell having the best P-CCPCH RSCP in one carrier as the serving cell is a UE's best choice for intra-frequency handover.
However, for inter-frequency handover cell quality criteria, selecting a cell having the best P-CCPCH RSCP is not always the best choice since a good P-CCPCH RSCP does not necessarily indicate low interference at slot 0 compared with other cells.
Thus, what is needed is a new decision algorithm by which to select a best cell during inter-frequency handover based on minimizing admission cost, and ideally, such an algorithm would also be of use in an inter-system handover decision to a TD-SCDMA system, i.e. from a radio access technology other than a TD-CDMA system (e.g. a GSM system) to a TD-SCDMA cell and carrier within the cell.